I recently added long vowels to my latin project. When spoken, long vowels are held a little longer than regular ones and are indicated by putting a little bar, called a macron, above the vowel: ā, ē, ī, ō, or ū. Java can handle these unicode characters if you specify "-encoding utf8" when compiling but apparently fit can not; so any fit tests that were comparing strings that had macrons were failing. I tried changing my code to convert long vowels into regular vowels when comparing but that was messy and didn't always work right since long and regular vowels are different phonemes. Eventually I decided to do away with fit and switch to JUnit. I lost at least two programming sessions to fiddling with fit, working around fit's limits, and finally reimplementing the tests in JUnit.
So even though I was really looking forward to using fit and I think it could solve many problems I've seen with the *Unit frameworks, for me, at least this time, the costs of using fit vastly outweighed the supposed benefits.